I haven't looked at the page but if your sales die off suddenly and you haven't changed the copy...

I'd look for a change in traffic. You're getting steady people but are they all the right people? Or do you have spurts of traffic that are good, while most of it sucks? Or...

I'll use a true story to illustrate...

A client came to me after the sales letter I wrote him had been kicking ass for year... He said, "I don't know what happened. The conversion has tanked. Used to convert day in and day out at around 4%. Now I'm lucky if I can break 1%.

I told him to look at his traffic and get back to me. He did and found out that a few of his new clickbank affiliates were spammers, sending him junk traffic. He dropped them and voila... back to 4%.